A gas turbine engine generally includes a fan and a core arranged in flow communication with one another with the core disposed downstream of the fan in the direction of the flow through the gas turbine. The core of the gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. With multi-shaft gas turbine engines, the compressor section can include a high pressure compressor (HP compressor) disposed downstream of a low pressure compressor (LP compressor), and the turbine section can similarly include a low pressure turbine (LP turbine) disposed downstream of a high pressure turbine (HP turbine). With such a configuration, the HP compressor is coupled with the HP turbine via a high pressure shaft (HP shaft), and the LP compressor is coupled with the LP turbine via a low pressure shaft (LP shaft).
In operation, at least a portion of air over the fan is provided to an inlet of the core. Such portion of the air is progressively compressed by the LP compressor and then by the HP compressor until the compressed air reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section through the HP turbine and then through the LP turbine. The flow of combustion gasses through the turbine section drives the HP turbine and the LP turbine, each of which in turn drives a respective one of the HP compressor and the LP compressor via the HP shaft and the LP shaft. The combustion gases are then routed through the exhaust section, e.g., to atmosphere.
The LP turbine drives the LP shaft, which drives the LP compressor. In addition to driving the LP compressor, the LP shaft can drive the fan through a power gearbox of an epicyclic gearing arrangement, which allows the fan to be rotated at fewer revolutions per unit of time than the rotational speed of the LP shaft for greater efficiency. The power gearbox rotatably supports a sun gear that is disposed centrally with respect to a ring gear and a plurality of planet gears, which are disposed around the sun gear and engage between the sun gear and the ring gear. The LP shaft provides the input to the epicyclic gearing arrangement by being coupled to the sun gear, while the fan can be coupled to rotate in unison with the carrier of the planet gears or with the ring gear, depending upon whether a star gearbox or a planetary gearbox is used. Each planet gear meshes with the sun gear and with the ring gear. One of the carrier or the ring gear may be held stationary, but not both of them. Each planet gear is rotatable on its own bearing that is mounted on a support pin housed within a planet gearbox, which is fixed to the peripheral region of the carrier of the epicyclic gearing arrangement. The shaft of the fan is rotatable on its own bearing that is housed in a sun gearbox, which is also called the power gearbox.
For any given gas turbine engine application, the planet gears are designed to provide a set reduction ratio between the rotational speed of the LP shaft and the rotational speed of the fan shaft. Because each planet gearbox that houses each planet gear is disposed within the flow path of the gas turbine engine, the challenge is to design on the one hand a reliable and robust planet gearbox that meets all flight conditions of the engine while on the other hand designing a planet gearbox that is compact sufficiently to fit inside the flow path in a way that does not require the entire engine size to be larger and heavier than otherwise would be needed in order to accommodate the planet gearbox.
Since a planetary gearbox is used as a speed reducer or increaser in transmitting power from component to component, gearbox efficiency is of primary importance. Various dynamic issues invariably will arise during the extended operation of the power gearbox. Accordingly, the ability of the bearings to tolerate and mitigate these dynamic issues can improve the capacity, life and reliability of the power gearbox and thereby lower the frequency of the engine maintenance. Additionally, providing proper lubrication and cooling to the planet bearings (i.e., the bearings that support rotation of the planet gear) that support the planet gears is necessary to maximize the life of the planet bearings and the load capacity of the planet bearings. Thus, any improvement in the tolerance of the bearings to deal with anticipated dynamic issues must not adversely affect proper lubrication and cooling to the planet bearing.